It is well known that cryogenic liquids may be used as a heat sink in association with a heat source such that an engine between the heat source and heat sink may be used to convert the thermal potential energy into work. Typical cryogenic liquids are liquid air, liquid ammonia, liquid nitrogen, liquid hydrogen, liquid helium, liquid carbon dioxide, liquid oxygen, as well as liquid hydrocarbons such as methane, ethane, propane, butane, and liquid natural gas.
The temperature difference between the cold cryogenic liquid, which acts as a heat sink and a heat source, such as water, ambient air, sea water, waste heat, e.g. heated air or water, usually provides a sufficient spread in temperature such that thermal potential energy is available for conversion into useful work if a system is properly designed to take advantage of the thermal potential energy. Heretofore, this source of thermal potential energy has largely been overlooked because of the ready availability of other more conventional energy sources, i.e., electrical energy, hydrocarbon fuels and coal. Presently, however, there is a shortage of electrical energy as well as hydrocarbon fuels.
There are many instances in which cryogenic liquids are transported from one location to another, e.g. liquid hydrogen, nitrogen, oxygen, and the like, and pumped from a storage vessel to a transportation vessel (boat, truck or the like) to another storage vessel or to distribution network. For example, liquid natural gas (LNG) is normally transported by tanker ship, tank car, freight car or pipeline from one location to another, an operation which requires energy, usually electrical, to power the pumps. Heretofore, this has not presented any major problems since the LNG was domestically supplied and, if transported domestically, was usually in gaseous form by pipeline networks.
The shortage of natural gas has resulted in the importation of relatively large quantities of natural gas, usually transported by boat and in a liquid state to reduce the total cost of shipment. In this case, the freighter tankers are loaded and delivered to an on-shore or off-shore facility for distribution locally or through a pipeline network to other areas. The on-shore or off-shore receiving facilities or terminals normally include storage facilities in which the LNG, liquified and at relatively low pressures on the tanker, is stored in the facility of relatively low pressures and in a liquid state.
With the increasing demand for natural gas and the discussion of more on-shore and off-shore LNG facilities for receipt of LNG brought into major user areas, there has been increasing concern both from an ecological and safety standpoint as to the location and operation of such facilities. From a safety and ecology standpoint, the ideal location of an LNG terminal is off-shore or on-shore in a sparsely populated area in which the facility design is such that ecological impacts are not deemed unreasonably adverse.
The problem, however, is that off-shore terminals or sparsely populated areas, otherwise acceptible, may lack the available energy needed for facility function. For example, if LNG is to be shipped from the on-shore or off-shore terminal to other locations, it may be vaporized for pipeline transmission at substantial pressures. If shipped as a liquid, it must be pumped into tank cars or freight cars. On the other hand, if the LNG is to be introduced into a user distribution network from any storage facility, it is vaporized and the pressure is increased substantially, e.g., 1200 psi.
Where handled as a liquid, the pressure of the LNG may be increased from storage tank pressure of between ambient and 30 psi to 1200 psi. This is usually accomplished by electrically driven pumps which may include multistage pumps or gear boxes driven by the motor to achieve the pump speeds needed for generating the high pressure. Where the LNG is vaporized to a gas, the LNG is flowed through a vaporizer to produce an output at 1200 psi and at 50.degree. F. by some application operation approximately 11/2%, depending on the system, of the LNG through put is maybe burned to produce the heat for the vaporizer. Thus, for example, if a particular facility ships 8 million dollars a day of gaseous LNG, the costs for vaporization may be substantial if LNG is burned to provide heat for the vaporizers.